The present invention relates to a lens unit and a compact image pickup module that are attached to a mobile telephone or a personal digital assistant (PDA) with a photographing function, an inexpensive, compact, and fixed-focus digital camera, or the like and include an imaging lens including multiple lenses and a lens holding member. In particular, the present invention relates to a lens unit and a compact image pickup module that use lenses in easy-to-manufacture shapes, is high in alignment accuracy or registration accuracy of the optical axis of each lens, and is easy to assemble.
A compact image pickup module installed in a mobile telephone or a PDA with a photographing function, a compact digital camera, or the like is ordinarily constructed by combining an imaging element holding member (hereinafter referred to as “CCD holder”) that holds an image pickup element (generally a CCD sensor), and a cylindrical lens-barrel that holds an imaging lens.
Such a compact image pickup module generally uses a CCD holder having a through hole and is assembled by fixing a CCD sensor to one end of the through hole, accommodating a lens-barrel holding the imaging lens from the other end of the through hole, and fixing the lens-barrel.
More specifically, as is well known, screw threads are cut into the inner wall surface of the CCD holder and the outer wall surface of the lens-barrel and are caused to mesh with each other. Then, focusing (focus adjustment) is performed by adjusting a distance between the light receiving surface of the CCD sensor and the imaging lens through adjustment of the screwing amount into the lens-barrel. After the focus adjustment is performed, the CCD holder and the lens-barrel are fixed to each other using an adhesive or the like when the compact image pickup module is a fixed focus module.
Also, as a result of technological advancement in recent years, the image pickup element, such as the CCD sensor, has a highly reduced size and increased resolution. Therefore, the imaging lens (lens unit) applied to the compact image pickup module is also required to have resolving power with which it is possible to separate thin lines at a level of 150 to 200 [1 p/mm] from each other. That is, an imaging lens having such high resolving power needs to be used as the imaging lens.
It is difficult to realize an imaging lens (lens unit) having such resolving power using a single lens. Therefore, an imaging lens (lens unit) having intended resolving power or precision is ordinarily realized using a combination lens obtained by combining multiple lenses with each other.
As schematically shown in FIG. 6, such an imaging lens (lens unit) produced by combining multiple lenses with each other ordinarily uses lenses 100 (100a, 100b, and 100c) having mutually different outer diameters and a lens-barrel 102 obtained by forming a lens holding portion whose size is changed stepwise so as to correspond to the respective lens diameters, and is constructed by incorporating the lenses into the lens-barrel 102 in the order of the lens 10c, the lens 10b, and the lens 100a in accordance with the lens diameters and fixing the lenses 100 to the lens-barrel 102 (see, for example, JP 2002-82272 A).
As a matter of course, in order to obtain a high-quality photography image, it is required that the imaging lens is fixed so that the optical axis of the imaging lens, that is, the lens-barrel is not inclined with respect to the light receiving surface of the CCD sensor.
When the optical axis is inclined, an imaging surface is also of course inclined, so a photographed image is not appropriately formed on the light receiving surface of the CCD sensor and image quality is degraded.
As described above, however, in the conventional compact image pickup module, the screw threads of the lens-barrel and the CCD holder are caused to mesh with each other, focus adjustment is performed by adjusting the screwing amount, and then the lens-barrel and the CCD holder are fixed to each other. With this construction, gaps inevitably exist between the screw threads, so the lens-barrel is inclined and the optical axis of the imaging lens is inclined.
As described above, the CCD sensor has a highly reduced size and increased resolution due to the recent technological advancement and therefore an imaging lens having high resolving power, with which it is possible to separate thin lines at a level of 150 to 200 [1 p/mm] from each other, is used as the imaging lens applied to the compact image pickup module. Therefore, even when the optical axis is slightly inclined, there occurs extremely significant image quality degradation.
Further, in an imaging lens provided with a plurality of lenses as described in JP 2002-82272 A, as a matter of course, in order to obtain a high-quality photography image, it is required that the optical axes of the lenses 100 incorporated into the lens-barrel 102 appropriately coincide with each other. In particular, in the compact image pickup unit, the imaging lens is required to have high resolving power as described above. Therefore, even when slight displacements (hereinafter referred to as “inter-lens eccentricity”) among the optical axes of the lenses 100 occur, there occurs extremely significant image quality degradation.
Between the lens-barrel 102 and the lenses 100, however, gaps (clearances) for incorporating the lenses 100 into the lens-barrel 102 exist, so the incorporated lenses 100 move in the lens-barrel 102 and the inter-lens eccentricity occurs.